1. Field of the Invention
The present invention relates to a liquid immersion oil having desirable characteristics with respect to transparency, viscosity, durability and the like. More particularly, it relates to a liquid immersion oil having low fluorescence which is suitable for use as a liquid immersion oil in fluorescence microscopes and ability which can regulate a refractive index throughout a wide range and is suitable for use as an optical liquid immersion oil in the field of optical measurements or other related fields.
2. Description of the Related Art
Liquid immersion oils are extremely widely used in the optical systems of microscopes, optical measurements and other related fields.
The reason that liquid immersion oils are often used in such fields is that, in comparison with an optical system not using such an oil, the use of a liquid immersion oil not only substantially reduces surface aberrations but also allows for increases in the magnification of the optical system by increasing the numerical aperture of the objective lens.
Such liquid immersion oils as Cedar oil or Cargill oil are well known in the prior art.
Cedar oil is oil obtained from oil of cedar wood being dissolved in toluene. However for example, when used during observations using microscopes, toluene gradually vaporizes with the passage of time. Therefore the oil not only loses its optical capabilities as a liquid immersion oil but also hardens as a result of polymerization due to oxidation reactions between oil of cedar wood and air.
Cargill oil entails the problem of toxicity to the human body since it contains large amounts of polychlorinated biphenyl (PCB).
Thus in recent years, the following oils which do not have the above drawbacks have been used as liquid immersion oils for microscopes: a liquid immersion oil based on benzylbutyl phthalate and chlorinated paraffin as disclosed in U.S. Pat. No. 4,465,621, a liquid immersion oil based on a liquid dien-polymer and liquid paraffin as disclosed in Japanese Patent Publication No. 4-13687, a liquid immersion oil based on a liquid hydrocarbon copolymer with, a diphenylmethane derivative or a bis-(.alpha.-alkylbenzyl)-alkylbenzene derivative or 2,4-diphenyl-4-dimethylphenylbutane as disclosed in Japanese Patent Publication No. 55-35053.
Furthermore in the optical systems of optical measurements and other related areas, 1-pentanol-1, 2-dibromopropane, methylsalicylate, nitrobenzene and the like have been used as immersion oils. A substance which has a refractive index required for use is optionally selected among these substances and is used.
However, the above oils have the low viscosity and can not be held as liquid immersion oils, and thus lack of liquid occurs when using. Furthermore a desirable refractive index is sometimes not achieved.
Thus mixtures of two or more compatible substances which have refractive indexes different from each other, such as mixtures of petroleum and turpentine oil, turpentine oil and 1, 2-dibromoethylene, 1-bromoethylene and clove oil, achieves a liquid immersion oil which has a refractive index required when using. And these mixtures are used as immersion oils.
Such liquid immersion oils such as those based on benzylbutyl phthalate and chlorinated paraffin, or based on a liquid diene-polymer and liquid paraffin, or based on liquid hydrocarbon copolymer with, a diphenylmethane derivative or a bis-(.alpha.-alkylbenzyl)-alkylbenzene derivative or 2,4-diphenyl-4-dimethylphenylbutane sufficiently satisfy several requirements for use as a liquid immersion oil such as refractive index, Abbe's number, resolution and the like. On the other hand however, such oils give off large amounts of fluorescence when excited by ultraviolet radiation (rays). As a result, observational accuracy is reduced when they are used as an liquid immersion oil for fluorescence microscopes.
Fluorescence microscopes are used in the fields of medical treatment field and biology to observe specimens by irradiating excited light such as ultraviolet radiation onto a specimen and observe the fluorescence given off by the observation object (cell, tissue of an organism or bacteria)
Particularly in recent years, much research has been made into fluorescence microscopes which detect the weak fluorescence given off by extremely small amounts of bacteria or cells. However large amounts of fluorescence may be given off when ultraviolet radiation excites a liquid immersion oil used in the optical system of fluorescence microscopes while detecting such weak fluorescence as above. Such fluorescence acts as noise during observation (detection) and thus reduces the accuracy of the observation (detection).
Furthermore liquid immersion oils created from the mixture of compatible two or more substances which have different refractive indexes from each other, such as mixtures of petroleum and turpentine oil, turpentine oil and 1, 2-dibromoethylene, and 1-bromoethylene and clove oil, entails the problem that turpentine oil and clove oil harden and undergo coloration due to oxidization. Furthermore the problem has arisen that many combinations contain substances which adversely affect the human body or the environment in general or which have low flash points.